gas-phase acidity 

https://doi.org/10.1351/goldbook.G02587
The negative of the Gibbs energy (ΔGr) change for the reaction:
in the gas phase.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1116 [Terms] [Paper]